1. Field of the Invention
The present invention relates to a molecular pump and, more particularly, to a turbo-molecular pump which is used for evacuating a vacuum vessel, for example.
2. Description of the Related Art
Molecular pumps such as turbo-molecular pumps and screw groove pumps are frequently used to evacuate vacuum vessels such as a semiconductor manufacturing system and an electron microscope which require high vacuum.
These molecular pumps have inlet ports provided with flanges adapted to be fixed to evacuating ports of vacuum vessels such as by bolts, respectively. An O-ring or gasket is interposed between the flange and the evacuating port of the vacuum vessel so as to keep air tightness between the molecular pump and the evacuating port.
Inside the molecular pump, there are provided a rotor section which is pivotally supported so as to be rotatable and which can be rotated at a high speed by a motor section, and a stator section fixed to a casing of the molecular pump.
In the molecular pump, the rotor section rotates at a high speed so that the rotor section and the stator section exhibit an evacuating effect. By this evacuating effect, a gas is sucked from the gas inlet port of the molecular pump and exhausted from a gas discharge port.
Usually, the molecular pump exhausts a gas in a molecular flow range (a range in which a vacuum degree is high so that the frequency of collision among molecules is low). To exhibit an evacuating ability in the molecular flow range, the rotor section is required to rotate at a high speed such as on the order of 30,000 revolutions per minute.
Meanwhile, in a case where some trouble has occurred during operation of the molecular pump so that the rotor section has collided with the stator section and the other fixed members within the molecular pump, the angular momentum of the rotor section is transmitted to the stator section, fixed members and the like so that a larger torque is instantly generated which rotates the entire molecular pump in a rotating direction of the rotor section. This torque also exerts a large stress to the vacuum vessel through the flange.
Thereupon, the following techniques have been proposed to mitigate such a shock.
[Patent-Related Reference 1] JP-A-1998-274189
[Patent-Related Reference 2] JP-A-1996-114196
Both of the techniques proposed in the patent-related reference 1 and the patent-related reference 2 are to provide a buffering mechanism at a flange disposed at a gas inlet port of a turbo-molecular pump.
FIG. 23 a flange having the buffering mechanism as proposed in the patent-related reference 1.
In FIG. 23, a flange 201 is provided at a gas inlet port of the turbo-molecular pump. The flange 201 is provided with a plurality of bolt holes 203 in elongated hole shapes on the same circle along an arc of the flange 201 and concentrically therewith. Contrary, the flange at the vacuum vessel side has the same outer diameter and inner diameter as the flange 201, and is provided with bolt holes in normal shapes (having cylindrical inner peripheral surfaces) arranged on the same circle concentrically with the flange itself at the vacuum vessel side.
The flange 201 and the flange at the vacuum vessel side are concentrically aligned with each other, the bolts 202 are then inserted through the bolt holes of them, respectively, and nuts are threadedly fitted over these bolts and then tightened, so that the turbo-molecular pump is fixed to the vacuum vessel.
Upon mounting the turbo-molecular pump onto the vacuum vessel, the bolts 202 are to be fixed at the ends of the bolt holes 203 in the rotation direction of the rotor. Then, in the case of a torque being generated which rotates the turbo-molecular pump in the rotation direction of the rotor when the rotor section is broken and touches the stator section and the like, the flange 201 slides (slips) in the rotation direction of the rotor so that the shock caused by the torque in the turbo-molecular pump can be buffered.
Further, the patent-related reference 1 also discloses a technique where each bolt hole (of circular cross section) of the flange 201 is formed to be sufficiently larger than the outer diameter of the bolt 202, and a buffering material is interposed between the bolt 202 and bolt hole 203.
The patent-related reference 2 describes a technique for absorbing the torque caused in the turbo-molecular pump by breakage of the rotor section and the like, by plastically deforming the bolts for joining the turbo-molecular pump to the vacuum vessel into an elbowed shape.
To plastically deform the bolts in the above manner, the bolt holes of the flange at the turbo-molecular pump side are formed into elongated hole shapes in the rotation direction of the rotor, and a thin plate portion in a pawl shape for deforming the bolt into the elbowed shape is formed near a bottom of each elongated hole.
When the structure for absorbing a shock by the flange portion of the turbo-molecular pump is used identically to the techniques disclosed in the patent-related references 1, 2, the safety of the turbo-molecular pump is enhanced. Further, the mounting strength between the flange portion of the turbo-molecular pump and the flange portion of the vacuum vessel side can be then reduced as compared with a case of absence of such a buffering mechanism (i.e., when the absorbing mechanism is absent, it is required to enhance the mechanical strength of the mounting portions so as to withstand an occurring torque, and required to enhance the mounting strength), and the manufacturing cost, working cost and the like can be reduced.
However, the patent-related reference 1 describing the bolt holes 203 formed into the elongated hole shapes presents a problem of complicated positioning (phasing) of the bolts on the installing job site. Also, there is a disadvantage that the shock-absorbing properties are changed depending on the tightening state of the bolts. Further, there is a problem of an increased cost, in case of using a buffering material.
Further, in the technique described in the patent-related reference 2, the shock-absorbing properties are changed depending on the natures (material, rigidity, property relative to shearing stress, and the like) of bolts to be used. It is thus desirable to specify a bolt for mounting, in case of guaranteeing a predetermined shock-absorbing property. Unfortunately, many kinds of bolts having the same shapes and different natures are distributed, so that the distribution, mounting and the like of turbo-molecular pumps are complicated in case of specifying the combination of turbo-molecular pump and bolts which are members different from each other. Also, when bolts of types different from specified ones are used, the used bolts are likely to rupture so that the turbo-molecular pump is dropped away from the vacuum vessel. Moreover, there is another problem of an increased machining cost, due to the thin plate portion in the pawl shape machined in the elongated hole.